The present invention relates to a laser source apparatus capable of continuously changing the oscillation wavelength, which is used in the fields of optical communications and precision measurement and, more particularly, to a tunable laser source apparatus which can continuously sweep the oscillation wavelength in a wideband by using an optical amplification element, e.g., a semiconductor laser (to be referred to as an LD hereinafter), in a wide wavelength range.
Recently, in the fields of optical communications and precision measurement, the application field of a heterodyne method, in which the frequency difference between two beams is measured in accordance with an electrical signal received upon superposing of the two beams, has been expanding.
In general, the frequency difference between beams that can be detected by a photodetector is electrically limited. In practice, the upper limit of this frequency difference is about 10 GHz.
If, therefore, one of two beams is used as a target beam to be measured, and the other is used as a reference beam, the frequency of the target beam must be tuned into a frequency within about 10 GHz.
When the high-order sideband spectrum of the target beam modulated with several GHz is to be measured, and the frequency and shape of an absorption line of atoms, molecules, and the like are to be observed, a laser source apparatus capable of continuously sweeping the oscillation wavelength near a desired wavelength is required.
As such a laser source apparatus, a tunable laser source apparatus called an external cavity laser is popular, in which light emerging from an optical amplification element having a wide gain band, e.g., a laser diode (LD), is fed back as light in a desired wavelength range through a wavelength selection element such as a diffraction grating disposed outside the optical amplification element, thereby causing laser oscillation within the wavelength range.
In such tunable laser source apparatuses, a wavelength selection element that is used most widely is a diffraction grating.
In this case, the wavelength to be selected is changed by changing the angle of the diffraction grating with respect to the incidence direction of light.
FIG. 9A shows the arrangement of a typical external cavity laser using a diffraction grating.
FIGS. 9B, 9C, 9D, and 9E show the principle of oscillation wavelength determination in the external cavity laser shown in FIG. 9A.
As shown in FIG. 9A, this external cavity laser is comprised of an LD 11 having one end facet 11b covered with an AR (Anti-Reflection) coating, lenses 13a and 13b, and a diffraction grating 21 disposed on the end facet 11b side.
The diffraction grating 21 can be rotated and translated. The diffraction grating 21 and the other end facet (without any AR coating) 11a of the LD 11 constitute an external cavity.
The arrangement of the diffraction grating in which the wavelength of light received from the LD and directly diffracted toward the LD by the diffraction grating is used as a selected wavelength is called a Littrow mounting.
The oscillation wavelength of an external cavity laser including a wavelength selection element is determined by two factors regardless of whether such a Littrow mounting is used.
The first factor is the wavelength that satisfies a resonance condition determined by the optical length of the overall cavity that causes laser oscillation.
In the optical cavity shown in FIG. 9B, let L be the optical length (to be referred to as the cavity length hereinafter) of the overall cavity, .nu. be the frequency of input light, P0 be the power of input light, and P1 be the power of output light.
As is known, if the speed of light is represented by c, the free spectral range (to be referred to as the FSR hereinafter) is expressed as FSR=c/2L.
As shown in FIG. 9C, each FSR includes a plurality of resonance frequencies at which the transmittance (output power P1/input power P0) becomes maximum.
When a given resonance frequency is n times the FSR, this frequency is called the nth-order mode.
Let a wavelength corresponding to a resonance frequency be called a resonance wavelength.
Another factor that determines the oscillation wavelength of the external cavity laser is the gain distribution that is band-limited by the diffraction grating shown in FIG. 9D or a wavelength selection element in general.
When an optical amplification element having a gain in a wide band, such as an LD, is used, the gain in the selected wavelength range of the diffraction grating can be regarded as constant. For this reason, the band-limited gain distribution can be regarded as identical to the selected wavelength spectrum of the diffraction grating.
A peak wavelength of a selected wavelength spectrum will therefore be simply referred to as a selected wavelength hereinafter.
As shown in FIG. 9E, therefore, the external cavity laser starts oscillation in one of the above modes which is positioned at the frequency at which the highest gain can be obtained.
In this case, in general, the selected wavelength does not coincide with the oscillation wavelength.
Consider sweeps of the oscillation wavelength.
FIGS. 10A to 10E show a change in oscillation wavelength in a state wherein the oscillation wavelength differs in change rate from the selected wavelength.
As the cavity length L and an input angle .theta. of light on the diffraction grating decrease, as schematically shown in FIG. 10E, the resonance wavelength and the selected wavelength shift to the short wavelength side, as shown in FIGS. 10A, 10B, 10C, and 10D.
When the resonance wavelength in the current oscillation mode differs from the selected wavelength by about 1/2 the FSR at this time, the oscillation wavelength shifts from the current oscillation mode to an adjacent mode to make a shift from the state in FIG. 10C to the state in FIG. 10D. As a result, the oscillation wavelength discontinuously changes.
This phenomenon is called a mode hop or mode jump.
To continuously change the oscillation wavelength throughout a wide band, a mode hop must be prevented by synchronously changing the resonance wavelength at which oscillation is being caused and the selected wavelength, i.e., simultaneously changing them while appropriately maintaining the relationship between the cavity length and the angle of the diffraction grating in the external cavity laser based on the Littrow mounting.
Conventional countermeasures against a mode hop have been based on two concepts.
A technique based on one concept aims at realizing a mechanism of capable of simultaneously changing the resonance wavelength and the selected wavelength while maintaining their relationship with one controlled variable to prevent a mode hop.
According to this mechanism, if the rotation center of the diffraction grating is set at a proper position, changes in the angle of the diffraction grating and cavity length can be kept at a predetermined ratio by only changing the rotational angle of the diffraction grating.
A technique based on the other concept aims at increasing the degree of freedom in control on the resonance wavelength and selected wavelength with a plurality of controlled variables.
According to the simplest example of this technique, the angle of the diffraction grating and the resonance wavelength, i.e., the cavity length, are independently changed on the basis of a combination of controlled variables obtained in advance in accordance with the oscillation wavelength.
In general, in an external cavity laser based on the above technique using one controlled variable, a laser medium, a lens, and the like are arranged, and they have wavelength dispersion characteristics, so the continuous wavelength sweep range is limited with the use of only the simple mechanism of keeping changes in the angle of the diffraction grating and cavity length at a predetermined ratio.
According to the technique using a plurality of controlled variables, although the continuous wavelength sweep range is not limited by the wavelength dispersion characteristics of elements in the external cavity laser, it is not easy to sweep the oscillation wavelength in a wide band while matching the oscillation wavelength with the selected wavelength with high precision by only setting the cavity length and the angle of the diffraction grating to values obtained in advance in accordance with the oscillation wavelength.
In addition, in the technique using a plurality of controlled variables, appropriate combinations of angles and cavity lengths as controlled variables must be obtained in advance by experiment and stored as parameters.
In each of the mode hop prevention techniques described above, although the oscillation wavelength can be continuously swept in a specific wavelength range, it is difficult to perform a continuous wavelength sweep throughout the entire oscillation band of the LD. In some cases, multi-mode oscillation occurs or no oscillation can be caused at a specific wavelength.
The technique using a plurality of controlled variables, in particular, performs wavelength sweeps, continuously relying on the mechanical precision at the initial adjustment and the parameters determined by experiment, and hence is susceptible to external disturbances, e.g., changes in cavity length and the like due to changes in ambient temperature.
In addition, each of the mode hop prevention techniques described above is susceptible to slight plastic deformation of components due to a shock and the like and changes over time.
Furthermore, in an adjustment stage for a conventional external cavity laser, wavelengths at which mode hops occur are checked while a movable portion supporting a diffraction grating or the like is moved to change the oscillation wavelength near a desired wavelength, and the laser is adjusted to increase the interval of the checked wavelengths, i.e., the continuous sweep range.
In this case, since there is no observation variable that indicates a direction of adjustment, a search for the maximum point of a continuous sweep range is performed by a trial-and-error method. This hinders an increase in productivity.